I bought a very large Agapanthus from the garden centre back in September. It's called White Heaven. When I bought it, it had a flower stem of nearly 36" which was nearly over and when it finally died in December I cut it off/down. I have kept it in our conservatory which faces West and has underfloor heating. Since then it has had quite a few leaves turn yellow, and I would appreciate it if you could tell me whether I am overwatering or underwatering it, but it now has a flower stem some 14" tall and the buds are beginning to develop and spread out.

Not sure about Agapanthus's grown under cover but is pretty normal for those outdoors to have their leaves tuen yellow and then lose their leaves.... interestingly all mine outdoors still have their leaves and they are still nice and green

Agapanthus's do like to be watered but don't like to stand in water so make sure that your pot is free draining.